1. Field of the Invention
The present invention relates to a process for preparing an optically active cyano compound. More particularly, it relates to a process for preparing an optically active cyanohydrin by a novel asymmetric cyanation reaction.
Cyanohydrin is a useful compound in a synthesis of an optically active compound with physiological activity such as a medicine and an agricultural chemical and used as a precursor of .alpha.-hydroxy acid or .beta.-aminoalcohol and an intermediate in the synthesis of pyrethroid.
2. Description of the Prior Art
An optically active cyanohydrin has been studied for a long time in view of an asymmetric synthesis, and many synthetic methods such as biochemical methods and a chemical asymmetric syntheses have been developed. However, none of them is industrially successful.
Among the biochemical methods, those utilizing an enzyme in a stereospecific addition of hydrogen cyanide to an aldehyde include (1) a method using emulsin [Fermentforschung, 5, 334 (1922)] and (2) a method using D-oxynitrilase [Angew. Chem., 77, 1139 (1965)], and enzymatic recemic resolution includes (3) a method comprising enzymatic asymmetric hydrolysis of an ester of racemic cyanohydrin with a carboxylic acid which is prepared by a usual chemical synthesis.
Amont the chemical asymmetric synthesis, those comprising asymmetric addition of hydrogen cyanide to an aldehyde in the presence of a basic asymmetric catalyst include (4) a method using a natrual alkaloid such as quinine and quinidine [Biochem. Z., 249, 241 (1932)], (5) a method using optically active polyaziridine [Bull. Chem. Soc. Japan, 38, 354 (1965)], (6) a method using an optically active quaternary ammonium salt phase transfer catalyst [Tetrahedron Letters, 2171 (1979)] and (7) a method using a cyclic peptide including hystidine [J. Chem. Soc., Chem. Comm., 229 (1981)]. Other known methods are (8) a method comprising reacting trimethylsilylcyanide with an optically active acetal of an aldehyde followed by oxidization to cause .beta.-elimination to asymmetrically produce a cyanohydrin [Tetrahedron Letters, 591 (1984)] and (9) a method comprising forming an inclusion compound of brucine and racemic cyanohydrin and accelerating conversion of racemic cyanohydrin to thermodynamically stable one [Chemistry Letters, 661 (1983)].
Although many methods for the synthesis of optically active cyanohydrin have been studied and proposed as described above, all of them have their own drawbacks. For example, the methods (1), (4), (5) and (6) produce a desired product with very low optical purity, the method (2) produces the product with good optical purity but only D-form ((R)-configuration), and the method (3) has difficulty in recovering an unreacted enantiomer. In the methods using the basic asymmetric catalyst, generally the produced cyanohydrin tends to be racemized by the basic catalyst and optical purity of the product decreases as the conversion is increased. Further, the production of the catalysts used in the methods (5) (6) and (7) is difficult.
The method (8) requires troublesome steps and consumes an expensive asymmetric source, and the method (9) can produce only a specific kind of cyanohydrin.